Issue #467: Add support for enabling modules and themes via bee cim.

[quicksketch]

Fixes #467

Backdrop 1.32.0 will be able to enable modules and themes via config after backdrop/backdrop-issues#5163.

This PR updates the bee cim command to take into account the new system.extensions.json config file and the special treatment needed to support it.

To test:

• Make sure your local is running the latest version of Backdrop 1.x from a Git checkout (i.e. 1.32.x-dev version)
• Run bee updb if needed to create the system.extensions.json file.
• Export current configuration with bee cex
• Find your staging directory and modify the system.extensions.json file. In my testing I added a line for contact: true to enable Contact module.
• Run bee cim to import the modified system.extensions.json file.
• Confirm Contact module has been enabled.

[jenlampton]

Run bee updb if needed to create the system.extensions.json file.

How does one generate a system.extensions.json file? I would like to test this.
My site has no available updates to run (I am on 1.32.1 and my install version was 1.31.0).

edit: I figured it out. Nevermind, and thankyou!

[yorkshire-pudding]

Run bee updb if needed to create the system.extensions.json file.

How does one generate a system.extensions.json file? I would like to test this. My site has no available updates to run (I am on 1.32.1 and my install version was 1.31.0).

@jenlampton - Hmm - that does seem odd - I'm sure I had the update hook run when I updated; I did update in the UI as Bee cannot (yet) update the files, though when I deployed to the staging then bee updb worked. The hook was system_update_1103()